1. Field of the Invention
The present invention relates to a nitrogen oxides purification device, in particular, to a nitrogen oxides purification device improving fuel consumption.
2. Description of Related Art
Exhaust gas discharged from an internal combustion engine of an automobile and the like contains components such as carbon monoxide (CO), hydrocarbon (HC), nitrogen oxides (NOx), or particulate matters (PM). Reduction of these components has been considered.
In order to reduce these components, a lean-burn system that reduces the amounts of these components discharged from the engine by performing an operation with a lean air-fuel mixture may be employed or a method of decomposing and purifying these components contained in the exhaust gas into harmless components with the use of a catalyst such as a noble metal may be employed. The lean-burn system reduces a production of the harmful components. Usually, the lean-burn system also uses a catalyst to decompose and purify the harmful components.
The automobile engines can be generally categorized into gasoline engines running on gasoline and diesel engines running on diesel oil. It is known that the diesel engine discharges a larger quantity of the particulate matters than the gasoline engine. Usually, the particulate matters are trapped with a filter catalyst such as a diesel particulate filter and are eliminated from the exhaust gas.
The lean-burn system increases an air-fuel ratio (A/F ratio) from a stoichiometric air-fuel ratio. Thus, the lean-burn system reduces a fuel injection quantity to reduce the harmful components discharged from the engine. The lean-burn system reduces an environmental load by reducing the harmful components discharged from the engine. The lean-burn system has been employed in the diesel engine in addition to the gasoline engine.
Usually, the vehicle has a catalyst device using a noble metal such as an oxidation catalyst, a reduction catalyst or a three-way catalyst to decompose and purify the above-mentioned components. Decomposition and purification of the carbon monoxide and the hydrocarbon are advanced by an oxidation reaction, and decomposition and purification of the nitrogen oxides are advanced by a reduction reaction.
Particularly in the diesel engine, the harmful components are eliminated by a catalytic metal after the particulate matters are eliminated by a filter catalyst. Therefore, there is a possibility that temperature of the exhaust gas becomes lower than activation temperature of the catalytic metal when the exhaust gas is purified by the catalytic metal. The harmful components cannot be purified by the catalytic metal if the exhaust gas temperature becomes low.
Therefore, nitrogen oxides purification devices for decomposing and purifying the nitrogen oxides contained in the exhaust gas have been developed. A certain nitrogen oxides purification device has a fuel injection device, platinum (catalytic metal) located downstream of the fuel injection device, and a nitrogen oxides purification catalyst located downstream of the platinum. The fuel injection device injects the fuel of the engine into the exhaust gas containing the nitrogen oxides. This nitrogen oxides purification device injects the fuel into the exhaust gas and decomposes the fuel with the platinum located downstream. Thus, the nitrogen oxides purification device increases the temperature of the exhaust gas and the nitrogen oxides purification catalyst with heat, which is generated in the progress of the decomposition reaction, over the temperature enabling exertion of the catalytic activation. Thus, the nitrogen oxides purification device decomposes and purifies the nitrogen oxides.
However, this type of nitrogen oxides purification device has to consecutively inject the fuel into the exhaust gas during the operation of the engine. As a result, fuel consumption is increased.